Pondweed bubbling under a lamp is one of the best-known IGCSE Biology practicals, and Paper 5 and Paper 6 keep coming back to it. By hand, put the heat shield in place, slide the lamp to each distance on the metre rule, wait for the plant to adjust, then run the stopwatch and tap the tally counter for every bubble. Record your own counts and temperatures, plot them, and explain the shape of your graph. Every attempt uses a different plant. Allow about 35 minutes.
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Headings with units, all six distances read correctly from the metre rule, whole-number bubble counts that match what really rose in each timed minute, temperatures to the nearest 0.5 °C, one-minute counts started after a one-minute wait, and the heat shield in place throughout.
Distance on the x-axis and rate on the y-axis with units, a y-axis scale that uses more than half of the grid, and every point plotted to within half a small square.
The trend described from your data, light intensity as the limiting factor further from the lamp, and another factor (carbon dioxide or temperature) limiting close to it.
Why the heat shield and the wait matter, why counting bubbles is inaccurate and how to measure the volume of gas instead, the control variables, and a safety precaution.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.